System and method for regulating pressure in an automatic transmission

ABSTRACT

An apparatus and a method for controlling the pressure in an automatic transmission with an electric pump. The electric pump supplies and removes hydraulic fluid from the automatic transmission in response to various conditions. The addition of an electric pump to control pressure within the transmission allows the mechanical pump driven by the engine to be reduced in size and improve fuel economy.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a hydraulic system for controlling the pressure of hydraulic fluid supplied to an automatic transmission of a vehicle, and, more particularly, to a hydraulic system that uses an auxiliary pump to regulate pressure supplied to an automatic transmission actuator.

[0002] Manufacturers of vehicles continually strive to improve fuel economy of vehicles while lowering the cost of a vehicle. Many approaches are used to accomplish these goals, but one area that has not been successfully addressed is the hydraulic system in an automatic transmission.

[0003] The basic structure and operation of hydraulic systems in automatic transmissions have not changed significantly since the advent of the automatic transmission. The engine drives a pump to create a fluid pressure, and a shift logic selectively communicates the fluid to a clutch assembly. As shown in FIG. 1, pressure regulation in the transmission is generally controlled by spool valves 100 which bypass the hydraulic fluid provided by the pump 10 to the sump 12 when the pressure exceeds a specified bypass pressure. The bypass pressure is varied by a regulation signal which is either mechanically (throttle cable) or electronically (solenoid) controlled.

[0004] The disadvantage to the above system is that the pump 10 is that the pump is driven by the engine so that the pump is producing hydraulic power whenever the engine is running. The constant driving of the pump 10 creates a drag on the engine and reduces fuel economy. Another disadvantage to these pumps 10 is that the displacement of the pump must be sized to meet the maximum hydraulic power demands including the high pressure demands required by heavy acceleration while the hydraulic fluid in the transmission is hot, due to the low volumetric efficiency of the pump at higher temperatures. Thus, during most operating conditions, the pump 10 creates a larger draw on engine power than is necessary. This draw reduces fuel economy and available operating power. Another disadvantage to these systems is that the pressure regulation system, including the spool valves 100, is expensive and adds parts to the transmission.

SUMMARY OF THE INVENTION

[0005] In view of the above, the present invention is directed to a hydraulic system having a second pump to regulate pressure. Generally, the hydraulic system includes a first pump driven by the engine and a second pump separately controlled by a pressure regulation system. The addition of a second pump allows the first pump to be reduced in size, thereby increasing fuel economy because the first pump is no longer sized for maximum hydraulic power demands. The first and second pumps provide fluid to an outlet conduit creating a pressure that is used by the automatic transmission to engage clutch assemblies. The regulation of pressure within the hydraulic system by the second pump allows the elimination of expensive and complicated pressure regulation systems that use valves.

[0006] More particularly, the present invention is directed to a hydraulic system in which the pressure provided by the second pump is variable so that the second pump may increase, maintain, or decrease the pressure provided by the hydraulic system. More specifically, this regulation of pressure is achieved by controlling the torque applied by an electric motor to the second pump. At certain pressures and torques the second pump may rotate backward, remain stationary, or rotate forward to adjust the pressure provided by the hydraulic system. An additional benefit to using a second pump attached to an electric motor is that the system may be configured to generate electricity from the supply of excess fluid generated by the first pump, driven by the engine.

[0007] Further scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present invention will become more fully understood from the detailed description given here below, the appended claims, and the accompanying drawings in which:

[0009]FIG. 1 is a schematic representation of a prior art hydraulic circuit;

[0010]FIG. 2 is a schematic representation of a transmission having a hydraulic system embodying the present invention;

[0011]FIG. 3 is a schematic representation of the pressure control system of the present invention; and

[0012]FIG. 4 is a schematic representation of an alternative pressure control system having a closed loop control system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] The present invention is generally directed to a system and method for controlling fluid pressure communicated to a transmission actuating assembly. With reference to the drawings, a vehicle (not shown) includes a power source, such as the illustrated combustion engine 18, drivably coupled to a transmission 8. The transmission 8 includes a hydraulic system 6 supplying fluid to a transmission actuating assembly 4. The transmission actuating assembly 4 includes a hydraulically actuated clutch assembly 16 and a conventional shift logic 14. The shift logic 14 receives fluid from an outlet 5 of the hydraulic system 6 and directs the fluid to the proper clutch assembly 16, creating an actuating pressure against the clutch assembly in a manner generally known in the art. The engaged clutch assembly 16 allows the transmission 8 to transfer an input torque from the engine 18 to the drive wheels (not shown).

[0014] The hydraulic system 6 provides fluid to the outlet 5 to create a fluid pressure within the transmission actuating assembly 4. As best illustrated in FIG. 2, the hydraulic system 6 includes a first pump 10 and a second pump 20 each hydraulically communicating with a fluid reservoir 12 and the outlet port 5. The first pump 10 is preferably driven, directly or indirectly, by the engine 18, as illustrated schematically by a shaft 19. The second pump 20 is hydraulically connected to the outlet 5 and reservoir 12 in parallel with the first pump 10. Accordingly, the pressure in the outlet 5 is, at any given time, affected by the operating states of the first and second pumps 10 and 20. A fluid filter 13 is illustrated as being interposed between the fluid reservoir 12 and the pumps 10 and 20. The filter 13 is located before the low pressure side of both pumps 10 and 20 so that the first pump 10 may draw fluid through the filter 13 as well as from line 124, providing efficient operation. As is typical with most transmissions, the fluid required by the transmission 8 is generally positive so that the fluid typically does not flow backward through the filter 13.

[0015] The transmission 8 also includes a pressure regulation system 7 which controls the drive torque on the second pump 20 and therefore the pressure provided by the hydraulic system 6 to the outlet 5. As best illustrated in FIG. 2, the pressure regulation system 7 includes an electric motor 21 and a servo amplifier 28. The servo amplifier 28 receives an input voltage 26 and a command current 80 from a control system 30 (FIG. 3) and creates a control output 29 which is supplied to the electric motor 21. The electric motor 21 creates a torque output in proportion to the control output 29 to drive the electric pump, directly or indirectly, such as by a shaft 23. The torque output to the second pump is unidirectional and greater than or equal to zero.

[0016] During operation, the engine 18 rotates the first pump 10 to supply fluid to the outlet 5 to create a fluid pressure. The magnitude of the pressure and corresponding flow rate from the first pump 10 is generally related to the rotational speed of the shaft 19 and therefore the engine rpm. The pressure regulation system 7 monitors and controls the pressure in outlet 5 by varying the operating state and performance of second pump 20. Depending on the fluid pressure at the outlet 5 and the desired pressure, the pressure regulation system varies the torque supplied to second pump 20 via shaft 23. For example, if the pressure regulation system 7 determines that more pressure is needed at the outlet 5 than is being supplied by the first pump, the pressure regulation system increases the torque exerted by the motor 21. Conversely, if less pressure is desired at the outlet, the torque from the motor 21 is reduced. It should be appreciated that the second pump 20 rotates forward, backward, or not at all based on the relative magnitudes of the torque exerted by the motor 21 and the fluid torque, generally created by the first pump 10. For purposes of this application, the second pump 20 is described as being operable in a first, second, and third operating state. In the first operating state, the second pump is rotating in a first direction to contribute to the pressure in the outlet 5. In the second operating state, the second pump 20 is rotating in a second direction opposite the first direction to detract from the pressure in the outlet 5. In the third operating state, the second pump 20 is stationary, not contributing or detracting from the pressure in the outlet 5. With the above general description of the invention in mind, the structure and operation of the transmission will now be described in detail.

[0017] As shown in FIG. 2, the first pump 10 supplies fluid to the outlet 5. The first pump 10 includes a first input conduit 120 and a first discharge conduit 122. The first pump 10 is driven by the engine and the selection and use of such pumps for automatic transmissions is generally well known in the art. The first pump 10 is generally referred to as a mechanical pump and, as used in the present invention, may be of a size smaller than typically used in automatic transmissions due to the addition of the second pump 20. A smaller sized first pump 10 draws less power from the engine 18 and improves fuel economy. As in most transmissions, the first pump 10 is attached to a torque converter (not shown) disposed between the engine 18 and the transmission 8. The first pump 10 is generally connected to the torque converter so that the pump is driven directly from the engine power and not by the output of the torque converter, which is used as the speed input into the transmission. Therefore, whenever the engine 18 is running, the first pump 10 supplies fluid to the outlet 5. The first pump 10 has a hydraulic fluid supply rate that is related to the temperature of the hydraulic fluid in addition to engine speed. Temperature affects the volumetric efficiency of a pump, causing a pump to supply less fluid at higher temperatures. The fluid is drawn from the reservoir 12 by the first pump 10 and second pump 20 and supplied to the shift logic 14 which routes the fluid to the proper clutch assembly 16.

[0018] As noted above, the use of a second pump 20 permits the hydraulic system 6 to selectively supplement the fluid pressure provided by the first pump 10 or to use excess fluid to generate electricity. In the former condition, the fluid supplied by the second pump allows the mechanically driven first pump 10 to be reduced in displacement size, thereby reducing drag on the engine and improving fuel economy. The second pump 20 includes a second input conduit 124 and a second discharge conduit 126. The second pump 20 is bidirectional, allowing the pump to rotate in either direction to add or detract from the fluid supplied to the outlet 5. The second pump 20 generally rotates backward when the fluid torque of the pressurized fluid in the outlet 5 exerts a torque greater than the torque applied to the second pump. Therefore, when the second pump is supplying hydraulic fluid, the first and second pumps 10 and 20 operate in parallel. When the second pump 20 removes or releases hydraulic fluid from the outlet port 5, the second pump operates in series with the first pump 10 allowing fluid supplied from the first pump to be released from the outlet 5.

[0019] As illustrated in FIG. 2, the second pump 20 is physically interconnected with the first pump 10 so that excess fluid supplied by the first pump is releasable through the second pump and to the inlet conduit 120 of the first pump without needing to first pass to the reservoir 12. This arrangement, as illustrated in FIG. 2, allows greater hydraulic efficiency and eliminates the need to filter the fluid twice. More specifically, the input conduits 120 and 124 and the discharge conduits 122 and 126 are interconnected so that in the first operating state, the first pump 10 operates in parallel with the second pump 20 to draw fluid from the reservoir 12 and supply that fluid to the outlet 5. In the second state, the first pump 10 operates in series with the second pump 20 so that the fluid supplied by the first pump 10 may be returned by the second pump 20 to the inlet conduit 120 on the first pump 10. Of course, it should be readily recognized that the first pump 10 may be withdrawing a small amount of fluid from the reservoir 12 in the second operating state in addition to the fluid supplied by the second pump 20 due to the generally positive fluid requirement of the transmission.

[0020] The pressure regulation system 7 controls the direction and rotational speed of the second pump 20. The pressure regulation system 7 includes the control system 30 which is configured to determine the command current 80 associated with a desired pressure in the actuation assembly 4. The command current 80 is input into the servo amplifier 28 and combined with an input voltage 26 to create a control output 29 which causes the electric motor 21 to apply a forward torque via shaft 23 to the second pump 20. The torque exerted by the electric motor 21 may be equal to or greater than zero depending on the control output 29. The electric motor 21 is preferably a brushless motor so that the second pump 20 may rotate forward, backward, or not at all without damaging the motor as it applies the forward torque. A brushless motor 21 allows a unidirectional forward torque to be applied to the second pump 20 while the second pump is rotating backward without harming the motor. The ability of the second pump 20 to rotate in either direction allows for the second pump to regulate the fluid pressure supplied to the outlet 5.

[0021] A conventional servo amplifier is used to drive the stator of the brushless motor with three sin waves, one wave for each phase of the motor. More specifically, the servo amplifier 28 receives an input voltage 26 as a power source to drive the electric motor 21 while the command current 80 is a control signal to the servo amplifier. Servo amplifiers are well known in the art for the control of three phase brushless motors. The amount of torque produced by the electric motor 21 directly affects the amount of pressure differential of the second pump 20 and thereby the fluid supplied by the hydraulic system 6.

[0022] Another benefit of the present invention is that the electric motor 21 may be configured to generate electricity while the second pump 20 is rotating backward (second direction) during certain operating conditions, for example, at highway speeds where the first pump 10 is supplying fluid in excess of that needed by the actuator assembly 4. The generation of electricity by the electric motor 21 allows the vehicle to convert excess hydraulic power to electric power.

[0023] The command current 80 input into the servo amplifier 28 is determined by the control system 30. The control system 30, illustrated in FIG. 3, is configured to determine a desired pressure, a fluid pump compensation, a second pump compensation, a static compensation, and a dynamic compensation. These compensations, described in greater detail below, are calculated as the current associated with each compensation, which are then summed by summation module 100 to provide the command current 80. The control system 30 includes various modules to calculate these compensations and the currents associated with the compensations. Because the currents associated with the compensations are to compensate for various operating conditions within the transmission 8, the currents provided by the modules may range from positive to negative to permit adjustment of the command current 80 based on the operating conditions of the transmission 8. The control system 30 illustrated in FIG. 3 and described below is a non-limiting example of a representative system. Those skilled in the art could easily modify or change the system to incorporate other suitable techniques for controlling the torque provided by the electric motor 21 and the resulting pressure at the outlet 5.

[0024] The desired pressure is determined by a desired pressure module 40 which receives an input as to the throttle position 42 and the transmission operating condition such as the gear position 44. The desired pressure module 40 calculates the desired pressure similar to systems commonly used in the art and associates a desired pressure current compensation value with the calculated desired pressure. The desired pressure is typically the minimum required transmission pressure at the outlet 5 determined using the throttle position signal 42 and the transmission gear position signal 44, as well as the respective clutch gain for the specified transmission gear. The throttle position signal 42 is used as an indication of engine input torque.

[0025] A static compensation module 90 calculates the current required for the motor 21 to supply the torque necessary so that the second pump 20 holds a pressure equal to the desired pressure. Of course, because the pressure at the outlet 5 may be greater or less than the desired pressure, the second pump 20 may rotate in either direction even though the second pump is receiving a forward torque in order to hold the desired pressure. More specifically, the static compensation module 90 calculates the portion of the command current 80 that is required to be supplied to the electric motor 21 so that the electric motor 21 holds the desired pressure assuming a static pump with no friction. The static compensation current calculated by the static compensation module 90 and associated with a particular desired pressure is easily determined for various pumps from the known operating characteristics of the specified pump, such as the torque required to drive that pump to generate the desired pressure level.

[0026] The control system 30 also includes a first pump compensation module 50 which determines a first pump compensation representative of the output from the first pump 10 in view of the operating characteristics of the transmission 8. To calculate the first pump compensation, the first pump compensation module 50 determines how much hydraulic fluid is supplied by the first pump 10 using conventional parameters such as engine speed 52 and transmission fluid temperature 54. The engine speed 52 affects the first pump compensation because the engine speed 52 is related to how fast the first pump 10 is turning and therefore how much fluid may be supplied in a given time period to the outlet 5. As noted above, the temperature of the fluid in the transmission 8 affects the pumping rate and is represented by transmission temperature sensor 54. For example, when the fluid is hot, the pump volumetric efficiency decreases and less fluid is delivered. The desired pressure of the fluid is also related to the pump's volumetric efficiency in that at higher pressures and temperatures more hydraulic fluid may leak past the internal seal of the pump. To respond to various engine speeds, the first pump compensation module 50 may calculate the change in the fluid flow supplied by the first pump 10 as engine speeds vary, and the related open loop change in the current supplied to the electric motor to compensate for the various engine speeds. For example, as the engine speed increases, the first pressure compensation module anticipates the increase in pressure from the increased fluid flow from the first pump. Therefore, the present invention may respond quicker to pressure changes than a system that monitors the pressure directly.

[0027] The dynamic compensation is calculated by a dynamic compensation module 70. The dynamic compensation represents the amount of fluid that needs to be supplied to the clutch assembly 16 to engage a gear and/or change gears. For example, the dynamic compensation module 70 calculates flow changes, such as how much fluid is needed to engage an oncoming clutch assembly 16 in the actuator assembly 4 during a gear change. Dynamic compensation allows the control system 30 to adjust for future changes in fluid flow. For example, if the operator shifts the transmission from reverse to drive, enough fluid must be released by the hydraulic system 6 to engage the drive gear clutches. Therefore, the present invention may anticipate and adjust for flow changes to increase the speed with which the transmission engages. The dynamic compensation module 70 uses a delta gear 72 and delta PRNDL 74 to indicate the change of gear so as to calculate an accurate estimation of the flow required to change gears. The dynamic compensation module 70 then calculates an open loop feed forward compensation for flow changes that are expected in the operation of the transmission. This allows the control system 30 to rapidly respond to flow changes without requiring a high gain closed loop. The use of a high gain open loop provides greater stability of the control system 30, allowing lower safety margins and greater fuel economy.

[0028] The control system 30 also includes a second pump compensation module 60 which calculates a second pump compensation. The second pump compensation module 60 determines the current required from the motor 21 to overcome the drag torque of the second pump 20 and the motor 21. The drag of the second pump 20 and motor 21 is a function of the speed with which the pump and motor are rotating as well as temperature of the fluid. The faster the second pump 20 rotates, the greater the windage losses. The lower the temperature of the fluid the greater the drag on the second pump 20. The second pump compensation 60 is calculated to attempt to linearly relate the current supplied to the motor 21 to the desired pressure 40.

[0029] The control system 30 takes into account the currents calculated by desired pressure module 40, the first pump compensation module 50, the second pump compensation module 60, and the dynamic compensation module 70 to determine a desired command current 80 to be supplied to the electric pump. In the illustrated embodiment, the commanded current 80 is simply a summation of the calculated current compensations calculated by a summation module 100. Notwithstanding this representative illustration, those skilled in the art will appreciate that other factors or compensations may be used without departing from the scope of the invention. As illustrated in FIG. 3, the determination of the command current 80 uses a simple open loop method. It should readily be recognized that a closed loop feedback of the pressure in the transmission may be used to increase accuracy of the command current 80, as illustrated in FIG. 4, and as described in more detail below.

[0030] Two exemplary methods are described herein to control pressure in the hydraulic system 6. The first method is an open loop pressure adjustment through controlling the forward torque of the electric motor 21 (FIG. 3). If the hydraulic system 6 is providing a specified pressure to the outlet port 5, and it is desired to reduce the pressure, the control system 30 reduces the command current 80 to a command current 80 associated with the desired pressure. In calculating the command current 80 associated with the desired pressure, the control system 30 uses the modules 50, 60, 70, and 90 to calculate the first pump compensation, static compensation, dynamic compensation, and second pump compensation. At certain pressures, the electric motor 21 applies a forward torque to the second pump 20, but the second pump 20 rotates backward to release pressure in the system. The second pump 20 rotates backward when the fluid pressure in the conduit 126 creates a torque greater than that applied by the electric motor 21. As the pressure reaches the desired pressure, the electric pump 20 will rotate forward, backward or stop to maintain the desired pressure level. In most cases, due to the continuous fluid supply from the first pump 10, driven by the engine 18, the second pump 20 rotates backward. When it is desirable to increase the pressure in the system, the control system 30 increases the command current 80 to a command current 80 associated with the increased desired pressure so that a greater volume of fluid is supplied to the actuator assembly 4.

[0031] A second method of control is to use a closed loop control (FIG. 4) of the pressure in the system, along with the open loop method previously described. This second method includes a closed loop compensation module 110 which provides a current compensation to slowly increase the command current 80 based on the fluid pressure. More particularly, the module 110 slowly increases the command current if a pressure sensor 24 determines that the pressure is lower than desired and decreases the command current if the pressure is higher than desired.

[0032] As noted above, the second pump 20 may be rotating backward to relieve pressure in the system and return the fluid to the sump 12. This backward rotation may be used to generate electricity. More specifically, running the servo amplifier 28 such that the torque on the pump 20, applied to the motor as the pump 20 is rotating backward, generates the electricity. This method is similar to three phase single voltage power generation devices which are well known in the art for generating electricity.

[0033] The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims. 

What is claimed is:
 1. A hydraulic system for supplying fluid to a transmission clutch actuator comprising: an outlet having a fluid pressure; a first pump having a supply conduit and a discharge conduit; a second pump having a supply conduit and a discharge conduit, said discharge conduits of said first and second pumps hydraulically communicating with said outlet, said second pump being operable in a first operating state wherein said second pump increases the fluid pressure in said outlet and a second operating state wherein said second pump decreases the fluid pressure in said outlet; a pressure regulating system operably coupled to said second pump to control the operating state of said second pump.
 2. The hydraulic system of claim 1 wherein said pressure regulating system includes a torque source operably coupled to said second pump, said torque source communicating a unidirectional torque to said second pump during said first and second operating states.
 3. The hydraulic system of claim 2 wherein said torque source is an electric motor and wherein said pressure regulating system further includes a control system communicating a command current to control said torque said electric motor communicates to said second pump.
 4. The hydraulic system of claim 3 wherein said pressure regulating system further includes a servo amplifier receiving said command current and an input current, said servo amplifier communicating a control output to said electric motor.
 5. The hydraulic system of claim 2 wherein said electric motor generates electricity when said second pump is in said second operating state.
 6. The hydraulic system of claim 2 wherein said torque communicated by said electric motor to said second pump is less than a fluid torque exerted on said second pump when said second pump is in said second operating state, said torque difference causing said second pump to rotate backward in said second operating state.
 7. The hydraulic system of claim 1: wherein a fluid pressure in a second pump discharge conduit exerts a first torque on said second pump, wherein said pressure regulating system includes an electric motor communicating a unidirectional second torque to said second pump, said second torque having a magnitude controlled by said pressure regulating system, and wherein said electric motor is operably coupled to said second pump such that said second pump operates in said first operating state when said first torque is less than said second torque and in said second operating state when said first torque is greater than said second torque.
 8. The hydraulic system of claim 1 wherein said supply conduits and said discharge conduits form a hydraulic circuit and wherein said first pump and said second pump operate in parallel in said first operating state and in series in said second operating state.
 9. A hydraulic system for an automatic transmission of a vehicle having an engine, said hydraulic system comprising: an actuating assembly having a fluid pressure; a first pump having a first fluid supply; a second pump having a second fluid supply, said first and second fluid supplies combining to form said fluid pressure; a pressure regulation system including an electric motor communicating a unidirectional torque to said second pump to control said second fluid supply.
 10. The hydraulic system of claim 9 wherein said first fluid supply is a positive fluid supply to said actuating assembly and said second fluid supply is a positive fluid supply rate in a first operating state and a negative fluid supply in a second operating state.
 11. The hydraulic system of claim 10 further including a control system, said control system communicating a command current to vary said unidirectional torque, said unidirectional torque creating a second pump pressure, said second pump pressure being greater than said fluid pressure in said first operating state, and said second pump pressure being less than said fluid pressure in said second operating state.
 12. A method of controlling a fluid pressure in an actuator assembly of an automatic transmission of a vehicle having an engine, comprising: supplying a hydraulic fluid to the actuator assembly with a first pump to create a fluid pressure; determining a desired pressure; and controlling said fluid pressure with a second pump.
 13. The method of claim 12 further including the step of determining a hydraulic fluid supply rate of said first pump.
 14. The method of claim 13 further including the step of determining a drag torque of said second pump.
 15. The method of claim 14 further including the step of determining a current compensation associated with each of said desired pressure, said drag torque, and said first pump hydraulic fluid supply rate.
 16. The method of claim 15 comprising the step of determining a command current associated with said current compensation, said command current being the sum of said current compensations.
 17. The method of claim 16 comprising the step of supplying said command current to a servo amplifier, said servo amplifier providing a control output to a torque source coupled to said second pump.
 18. The method of claim 17 further including the step of providing said control output to an electric motor coupled to said second pump.
 19. The method of claim 18 further including the step of applying a forward torque to said second pump with said electric motor. 